The present invention relates to a housing construction for a transmission device, and more particularly relates to such a housing construction which can aid in the generation of a pool of lubricating oil in the lower part of the casing of the transmission device, which provides particularly good lubrication for a bearing through which rotational power passes between the outside and the inside of the transmission device. In particular, the present invention is suitable for application to a transmission system for a vehicle of the type in which, selectively, either the front wheels only can be powered, for use of the vehicle in relatively good road conditions such as on a paved road, or alternatively all four wheels can be powered, for use of the vehicle in relatively poor road conditions such as off the paved road surface. In such a vehicle, a power switching and transfer device is required which can switch the vehicle between single axle operation--i.e. front wheel drive only type operation--and double axle operation--i.e. front and rear wheel drive type operation or four wheel drive operation. Such a power switching and transfer device is subjected to heavy loads, and the problems of lubricating, in particular, the bearing which supports power output shaft for supplying rotational power to the rear wheels of the vehicle can become acute.
In detail, such a power switching and transfer device typically includes a power output shaft for outputting rotational power to the rear wheels of the vehicle, via a propeller shaft and a differential and rear axle assembly and so on, when and only when the vehicle is to be operated in four wheel drive mode, and this power output shaft is typically rotatably supported at its front end (with respect to the longitudinal axis of the vehicle) by a front end ball bearing fitted inside a forward portion of the housing of the power switching and transfer device and at its rear end by a rear end bearing or bush which is fitted inside a portion extending towards the rear of said housing. Thus, the front end of the power output shaft is selectively provided with supply of rotational power from some rotating member, such as an intermediate shaft, of the main transmission of the vehicle which powers the front wheels thereof, when powering of the rear wheels is also desired, and the rear end of the power output shaft is connected with regard to rotation to the front end of said propeller shaft. In order to provide stable support for this power output shaft, which typically is required to transmit great rotational torque and accordingly is required to be very securely rotationally mounted, the distance between said front end ball bearing and said rear end bearing is typically quite great.
Now, the lubrication for these bearings is conveniently provided by providing the intermediate shaft with an axially extending passage which communicates to the inside of a hole formed at the front end of the power output shaft. In its turn, the power output shaft is formed with one or more radially extending apertures, joining at their inner ends to said hole, which communicate to the outside of the power output shaft a little to the rearward direction of said front end ball bearing. Thus, when the power switching and transfer device is being operated to provide four wheel drive type operation, lubricating oil is supplied to the front end of this axial passage in the intermediate shaft, and this lubricating oil passes down the axial passage and to the hole in the front end of the power output shaft, and is then ejected through the radial passage or passages in said power output shaft as said power output shaft revolves, so that the majority of this lubricating oil is supplied to said front end ball bearing. Now, in the prior art, it has been relied upon that a sufficient amount of this lubricating oil will be transmitted so far as to reach the rear end bearing or bush, via the extension of the transmission housing supporting said rear end bearing, as to provide sufficient lubricating oil therefor; but this cannot be absolutely relied upon, and sometimes problems have arisen with respect to lubricating such a rear end bearing, leading in the extreme case to high wear and/or premature failure thereof. Particularly when the vehicle is being operated at low speed, or at low temperatures, this lubrication problem can be troublesome; and it is also aggravated in the period before the transmission lubricating oil has warmed up and while its viscosity is still great. Also, if the lubricating oil in the power switching and transfer device should become dirty and thick after a prolonged period of use of the vehicle without said lubricating oil being changed, then again the effectiveness of lubrication of the rear end bearing of the power output shaft can be undesirably deteriorated.